1. Field of the Invention
The present invention relates to lithium batteries. More particularly, the invention relates to organic electrolyte solutions including silane compounds and to lithium batteries employing the organic electrolyte solutions.
2. Description of the Related Art
Portable electronic devices, such as video cameras, cellular phones, and notebook PCs, are being developed to be more light-weight and to have higher performance. As such, much research into batteries used as driving power sources has been conducted. In particular, re-chargeable (secondary) lithium batteries have been actively studied since they can be rapidly re-charged and have energy densities per unit weight three times greater than those of conventional lead storage batteries, nickel-cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, etc.
Conventional lithium batteries are operated at high operating voltages, and thus, conventional aqueous electrolyte solutions cannot be used due to the vigorous reaction of the aqueous solutions with the lithium used as the anodes. Accordingly, organic electrolyte solutions obtained by dissolving lithium salts in organic solvents are used in lithium batteries. At this time, it is preferable to use organic solvents having high ion conductivities, high dielectric constants, and low viscosities. However, it is difficult to obtain a single organic solvent satisfying all these requirements, and thus, the use of mixed solvents has been proposed, such as a mixed solvent including both a high dielectric constant organic solvent and a low dielectric constant organic solvent, a mixed solvent including a high dielectric constant organic solvent and a low viscosity organic solvent, and the like.
When a carbonate-based non-aqueous polar solvent is used in a lithium secondary battery, excess charges are used due to the reaction between the carbon of the anode and the electrolyte solution during initial charging. Such an irreversible reaction forms a passivation layer, such as a solid electrolyte interface (SEI) film, on a surface of the anode. The SEI film serves to prevent further decomposition of the electrolyte solution and maintains stable charging/discharging. The SEI film also serves as an ion tunnel through which only lithium ions pass. That is, the SEI film prevents the cointercalation of lithium ions with the organic solvents that solvate the lithium ions and move together with the lithium ions into the carbon anode. This prevents the degradation of the anode structure.
However, the SEI film gradually cracks and delaminates from the surface of the electrode due to volumetric expansion and shrinkage of the active material during the charge/discharge cycles of the battery. As a result, the electrolyte directly contacts the active material, thus continuously decomposing the electrolyte. Once the SEI film cracks, the crack continuously extends during charging/discharging of the battery, thereby degrading the active material. In particular, when the active material contains metal, such as silicon, the active material degrades further due to great volumetric changes during charge/discharge cycles. Furthermore, the repeated volumetric shrinkage and expansion of the active material cause the agglomeration of silicon particles.
To address these problems, various compounds having characteristic structures capable of making denser and more rigid SEI films have been proposed. For example, a method of preventing the decomposition of a solvent has been proposed whereby a vinylene carbonate derivative is added as an additive to an electrolyte solution and a film is formed on the surface of the anode through a reduction/decomposition reaction of the additive. Vinylene carbonate has a structure similar to ethylene carbonate but contains a double bond. Thus, vinylene carbonate receives electrons at high voltages, disrupting the double bond in the vinylene carbonate molecule, thereby creating radicals and initiating polymerization. That is, vinylene carbonate provides a polymer through an electrochemical polymerization reaction. The thus-prepared polymer forms a nonconductive film on the surface of the anode, thereby more effectively preventing contact between the anode and the solvent.
Electrolyte solutions including vinyl sulfone, etc. have also been proposed. When a double bond in the vinyl sulfone, etc. is disrupted by reduction, radical polymerization is initiated, thereby forming a film on a surface of the anode.
Organic electrolyte solutions for lithium batteries including at least one silane compound as an additive have also been proposed.